fsync(2) and on-disk write-back cache
Jeremy Chadwick
freebsd at jdc.parodius.com
Tue Aug 31 16:08:43 UTC 2010
On Mon, Aug 30, 2010 at 06:58:42PM -0400, Martin Cracauer wrote:
> I always assumed the answer to this question is "of course":
>
> When doing an fsync (waiting for the commit), do we actually tell the
> disk to flush the on-disk write-back cache (if that is in use) to the
> platters?
>
> I just went down some code paths in both FreeBSD and Linux and in both
> cases the paths for fsync quickly disappear in the generic
> block-by-block flushing code that is also used for regular (non-fsync)
> flushing. I didn't see anything aware of the on-disk cache.
I don't have an authoritative answer to your question, but this thread
seems to imply there's a relation between fsync() and an intentional
disk flush (BIO_FLUSH). I'm sure when BIO_FLUSH is called depends on
the filesystem as well.
Funny timing this topic, given a post over in -stable:
http://lists.freebsd.org/pipermail/freebsd-stable/2010-August/058525.html
--
| Jeremy Chadwick jdc at parodius.com |
| Parodius Networking http://www.parodius.com/ |
| UNIX Systems Administrator Mountain View, CA, USA |
| Making life hard for others since 1977. PGP: 4BD6C0CB |
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